I know there can be issues with pituary gland or thyroid function that can affect a woman's breastmilk production (either at birth or later on). Or if a woman has formerly had breast surgery (for cosmetic or medical reasons) that can also prevent milk production. I believe hormone imbalances can also affect milk production. I'm sure there are other medical reasons that I don't know about.

This would be the very small percentage of mothers that are physically unable to bf. Other women have problems with bf, for so many reasons, which then affects supply, etc, etc, but they are physically capable of bf in that their breasts produce milk.

I too always thought women in the jungle manage to do it. Only I used to think, if I had a baby a hundred years ago I would have learnt how to bf because there was no alternative!